Agree. For thin slabs or soft stones I prefer full support under the slab. Plywood is pretty stable, it really doesn't expand or contract in its thickness. Slabs are usuallly set over the ply base using dollops of silicone. Just have the stone setters spread silicone around the location of the plumbing so when they set the slab you'll have a thin setting bed of silicon between the slab and the ply. The silicon will lock the ply to the marble, preventing any vertical wiggle that might cause the plumbing to unthread and loosen. Only other concern would be the length of the threaded nipples on the plumbing fixtures, they'd need to be longer than the thickness of stone and plywood so a nut can be threaded onto the nipple. If the nipples are too short, there are extensions that could be used. If I have to cut the ply away? I'll not remove an entire section, just what is needed. Example, if I needed three holes, each 1-1/2" in diameter and centered 8" apart. I won't cut a large rectangle away. I'll cut three holes in the ply, say 3" diameter holes, centered 8" apart. Then drill three 1-1/2" holes in the marble. Soft/thin stones need all the support you can offer.

I agree. This slab install we polished the ends as well - don't you love the clear look trim on this fireplace! This install was 6 pieces of tarvertine installed onsite. If you look close at the fireplace in this picture you will see we polished the top stone to lap the skinny piece. Travertine is soft and the quality of the sealers and filler varies. The look of this install was enhanced because we ordered a 40% waste factor. We Cherry picked the stone and spent 4 hours doing layout. The extra waste stone will become a laundry room floor. What type of fireplace is it? Make sure to test the effects of heat on your tile. Place a few in front and bring the fire up to full heat for a number of burn cycles. Get one tile set loosely above the fireplace for the same test. Test with the sealer as well. I case you didn't know - I build a lot of fireplaces - I just don't talk about it. I hear Mike Holmes next issue will talk about fireplace renovations. Lets see what the "Experts" have to say... ;)

Bill V - I found one of your comments in an old GW post from 2008. I copied and pasted below. So I'm thinking unsanded. If I am wrong, please let me know! Thank you so much for all of your help! As for the explanation of the different grouts, I just posted this in an FAQ thread on the gallery side of the bathrooms forum. Here it is, copied and pasted: Although there are others, for all intents and purposes, there are two kinds of grout-- portland cement based, and epoxy. The portland cement based grouts are the conventional grouts that have been around for millenniums. Although in the last few decades, they've been modified with latex and other polymers to make them stronger and more resistant to mold and mildew, they're basically the very same grouts that have been used since Greek and Roman days. There are two kinds of portland cement based grouts. One is sanded, and the other unsanded. The only difference between the two is, as their names imply, the sand. The ONLY thing that determines which grout should be used is the joint size. NOT the glaze, NOT aesthetics, NOT the material (ceramic vs. glass or polished marble), NONE of those. I'll repeat-- the ONLY thing that determines which is used, is the joint size. Anything under an 1/8" takes unsanded grout. Anything 1/8" or bigger, you use sanded grout. If you use unsanded grout in larger joints, the cement in the grout will shrink way too much as the water evaporates out of it, and the joints will end up shrinking and cracking bigtime. If you try using sanded grout in smaller joints, the grains of sand will literally clog the top of the joint, and not allow the grout to get down INTO the joint, and the grout will flake off in a matter of days.

It used to be that 1/16th was called out for rectified tiles. With the new specs for manufacturing, you start getting more variations in tile size. Even in rectified, believe it or not. The new installation recommendation, which has actually been around for a few years, is to have the grout line 3-times the size of the variation within the tiles. This usually comes down to an 1/8th inch grout line. Lippage can also be an issue based on the pattern. A stacked pattern might not show lippage whereas a running bond could. The larger the tile, the more chance you have of lippage with offset patterns. That's all due to the tiles not being perfectly flat. A slight warp, pillow, or pringing (like the pringles potato chip). The grout needs depth. Otherwise it'll just pop out. So a 1/32nd gap with a beveled edge, you may not have durability, because the installer may not pack the joint (if there even is an actual joint) with grout. So: Not to dump on the guy who plans on going 1/32nd...but I'd nix that. Someone who tells me they're going with 1/32nd, I think they're just going to lightly butt the tiles together and get whatever grout penetration they can. And it'll likely be nothing more than topical grout with very little actually packing whatever gap there would be between the tiles. With the new specs, 1/8th inch is the new standard. You could still go 1/16th if the tiles and pattern allow for it, the installer is comfortable with it, and it's called out in the specs...but I'd not advocate going less.